Latest / Hardware Startups with Fexingo: Devices, Robotics, and Manufacturing Tech Companies / How Hardware Startups Use Wave Soldering for Through-Hole Assembly
Transcript
- Lucas: Luna, when was the last time you thought about wave soldering? I mean, really thought about it. Luna: Probably never. I mean, I know it exists, but in the age of pick and place and reflow, it feels almost like a relic. Lucas: That's what I thought too until I visited a small robotics startup in Portland last month. They were assembling motor controllers — about ten thousand units for their first production run. And their entire board design was surface-mount, except for these big electrolytic capacitors and a terminal block that had to be through-hole because of the current load. Luna: Right, so they couldn't avoid through-hole entirely. What did they do — hand solder each one? Lucas: That was the initial plan, actually. They estimated hand soldering would take about three minutes per board for those few components. At ten thousand boards, that's five hundred hours of labor. And with two assemblers, that's over a month of solid work, not counting rework. Luna: Ouch. So they looked at wave soldering. Lucas: Exactly. They sent the boards to a local assembly house that had a wave soldering line. The whole batch — ten thousand boards — went through in about four hours. The cost was only two thousand dollars. Compare that to the labor cost of hand soldering, which would have been something like fifteen thousand. Luna: That's a no-brainer on cost. But I'm guessing wave soldering isn't as simple as just turning it on. What kind of yield did they get? Lucas: First pass was about ninety-one percent. Not terrible, but not great for a startup on a tight timeline. The main defects were solder bridges between the capacitor leads and some incomplete joints on the terminal block — what we call 'solder shadow' where the board is moving through the wave. Luna: Solder shadow — that's when the component itself blocks the solder wave from reaching the leeward side of the lead, right? Lucas: Exactly. The board travels over a standing wave of molten solder, and if the component is tall enough, it creates a wake — just like a rock in a river. The leeward side of the lead doesn't get proper wetting. In their case, the terminal block was about fifteen millimeters tall, and they were running the board with the long axis parallel to the wave direction. Luna: So they rotated the board ninety degrees? Lucas: They did. They also adjusted the preheat temperature profile. The original profile was heating the board to about one hundred degrees Celsius before the wave. They increased it to one hundred thirty, which helped the flux activate more uniformly and reduced thermal shock. After those two changes, the yield jumped to ninety-seven percent on the next batch. Luna: Ninety-seven percent is solid. So the lesson is: wave soldering isn't just about having the machine; it's about understanding the physics of the wave and the board design. Lucas: Exactly. And that's the sweet spot for a hardware startup. You don't need to own a wave soldering machine — a decent benchtop unit is around fifteen thousand dollars, and a full production line is fifty to a hundred thousand. But you do need to know enough to specify the right parameters when you work with an assembly house. Luna: Let's talk about those parameters. What are the levers you can pull? Lucas: The big ones are: solder temperature, conveyor speed, preheat temperature, flux type and application rate, and board orientation. Solder temperature is typically around two hundred fifty to two hundred sixty degrees Celsius for leaded solder, and a bit higher for lead-free — closer to two hundred seventy. Conveyor speed determines how long the board is in contact with the wave; typical speeds are one to two meters per minute. Luna: And flux — that's the chemical that cleans the surface and promotes wetting. Is that applied before the wave? Lucas: Yes, usually a spray fluxer applies a thin, even coat to the bottom of the board before preheat. The flux dries and activates in the preheat zone, so by the time the board hits the wave, the oxide layer on the copper and the component leads is reduced, and the solder can form a good intermetallic bond. Luna: I've heard a lot about no-clean fluxes. Do startups use those? Lucas: Almost exclusively. No-clean flux leaves a residue that is minimally conductive and doesn't need to be washed off, which saves a whole cleaning step. But it's not completely inert — if you have high-impedance circuits or sensitive analog signals, the residue can cause leakage currents over time. For most motor controllers or IoT devices, though, no-clean is fine. Luna: What about lead-free? Is that the default now for most wave soldering? Lucas: Pretty much. The RoHS directive has pushed the industry to lead-free alloys, typically SAC305 — that's tin, silver, and copper. But there's a catch: SAC305 has a higher melting point and is more brittle than traditional tin-lead solder. For products that undergo thermal cycling or vibration, like automotive or aerospace, that brittleness can be a problem. Some startups use a slightly different alloy, like SAC105 with a bit of bismuth, to improve reliability. Luna: So there's a trade-off between environmental compliance and reliability. How do you decide? Lucas: It depends on the application. If your product lives indoors and doesn't see extreme temperature swings, lead-free is fine. But if you're making something that goes on a drone or a factory floor, you might want to do some thermal cycle testing. The good news is that wave soldering with lead-free alloys is very mature now — the process windows are well understood. Luna: Let's get back to the yield improvement story. After they rotated the board and adjusted preheat, they hit ninety-seven percent. What was the remaining three percent of defects? Lucas: Mostly small solder balls trapped under the board — what's called 'solder balling' from flux spatter. And a few incomplete fillets on the back side of some connectors. They solved the solder balls by adjusting the flux spray rate — they were using too much flux, which caused excessive spattering. They also added a nitrogen blanket in the wave zone, which reduces oxidation and improves wetting. That brought them closer to ninety-nine percent. Luna: Nitrogen blanketing — isn't that expensive for a startup? Lucas: It can be, but some assembly houses offer it as an option. The nitrogen reduces dross formation — that's the oxide that forms on the surface of the molten solder — which not only improves wetting but also reduces solder consumption. So the cost of nitrogen can be partially offset by less waste. For a high-volume run, it can be worth it. Luna: So the startup in Portland — did they eventually bring wave soldering in-house? Lucas: They considered it, but their volumes are still only a few thousand boards per month. At that scale, outsourcing to an assembly house is cheaper. The rule of thumb I've heard is: if you're doing less than fifty thousand boards per year, it's hard to justify owning a wave soldering machine, especially when you factor in maintenance, operator training, and the cost of dealing with dross and flux fumes. Luna: That makes sense. But I know a lot of hardware startups eventually want to bring manufacturing in-house to have more control. Is wave soldering a good first step? Lucas: It could be, but I'd argue that selective soldering is often a better fit for startups. Selective soldering uses a small nozzle to solder individual through-hole pins one at a time, programmed robotically. It's slower than wave soldering, but it's much more flexible — you can handle different board sizes and component heights without changing tooling. And the machine cost is similar to a benchtop wave soldering unit. Luna: Selective soldering is interesting. But doesn't it have a higher per-joint cost? Lucas: It does — maybe two to three cents per joint versus half a cent for wave. But for startups with mixed-technology boards that have only a few through-hole components, selective soldering can be more cost-effective overall because you avoid the setup time and the risk of damaging sensitive surface-mount components that are already reflowed. Luna: So the key is to design with the assembly process in mind from the start. If you know you're going to use wave soldering, orient all through-hole components in the same direction, leave enough clearance between them to avoid shadowing, and make sure the board can withstand the thermal shock. Lucas: Exactly. And that's where a good relationship with your assembly house pays off. They can review your design for manufacturability and suggest changes before you commit to the board layout. That startup in Portland had to respin their PCB because the original layout had the terminal block too close to a large capacitor — the shadow was inevitable. A simple DFM review would have caught that. Luna: What about the solder wave itself? Is it a single wave or a dual wave? Lucas: Most modern wave soldering machines use a dual wave. The first wave is turbulent — it has a lot of energy to force solder into tight spaces and around tall components. The second wave is laminar — it's smoother and cleans up any bridges or excess solder left by the first wave. That dual-wave approach is what makes wave soldering reliable for complex through-hole boards. Luna: And the board is transported on a conveyor with fingers that hold the edges, right? What about boards with odd shapes? Lucas: Right — pallets are used for non-standard boards. A pallet is a custom fixture that holds the board and protects the bottom side of any surface-mount components that are on the board. The pallet has cutouts only where the through-hole components are, so the solder wave only contacts those areas. That's critical for mixed-technology boards, because you don't want to re-melt the surface-mount solder joints. Luna: That's a good point. So the pallet adds cost — maybe a few hundred dollars per design — but it's a one-time expense. Lucas: Exactly. And for a startup with a single product, it's a small price to pay for reliable assembly. The Portland startup's motor controller board was a standard rectangular shape, so they didn't need a pallet. But they did need to add tooling holes for the conveyor fingers. Luna: Let's talk about the bigger picture. Wave soldering has been around for decades. Are there any new technologies that could replace it? Lucas: For through-hole, not really. Surface-mount is dominant, but through-hole is still necessary for high-power connectors, large capacitors, relays, and anything that needs mechanical strength. There's some work on conductive adhesives and press-fit connections, but wave soldering is still the most reliable and cost-effective method for volume production. I don't see it going away anytime soon. Luna: So for a hardware startup listening today, what's the one takeaway? Lucas: Don't ignore through-hole assembly. Design your board so that through-hole components can be wave soldered — orient them consistently, leave enough spacing, and consult your assembly house early. You can save months of labor and thousands of dollars. And if your volumes are still low, consider selective soldering as an alternative. Luna: That's solid advice. And it's exactly the kind of practical insight that makes this show valuable — especially for founders who are building hardware, not just software. Lucas: And you know, that's part of why we do this show ad-free. We want these conversations to be useful for what you're building or running, without any commercial interruption. If you find that valuable and want to support the show, the link is buy me a coffee dot com slash fexingo. No pressure, but every bit helps us keep the episodes focused on the practical stuff. Luna: Yeah, absolutely. It's a small way to keep the podcast independent and free of ad reads. Lucas: So back to wave soldering — one more thing I want to mention. If you're prototyping and you only need a few boards, you can actually simulate wave soldering with a solder pot. Just dip the board's bottom side into a small pot of molten solder. It's messy and not very controlled, but for proof of concept it works. I've seen startups do that to validate their through-hole design before committing to a full production run. Luna: That's a good tip. And it's cheap — a solder pot is maybe a hundred dollars. Lucas: Exactly. Hardware startups need to be scrappy. Wave soldering — whether outsourced, in-house, or simulated — is one more tool in the toolbox. Use it wisely, and it can save you a lot of time and money. Luna: Great episode. Let's do one on selective soldering next time. Lucas: I'm already making notes.